[50mhz] antenna stacking question

[Bill W5WVO]

>> This is a long post. If you're not interested in 6M propagation and yagi
theory, please delete. <<

Chris, AC6TA wrote:

> Remember that most of your contacts on 6M will be via E skip and that
> has a much higher take off angle than F2.

This is a bit misleading. The effective take-off angles for both F2 and Es
propagation are functions of the same variables: the refracting layer's
critical angle (which is itself a function of ionization intensity), the
desired path distance, and the operating frequency. Because the sporadic-E
layer is much lower than the F2 layer, single-hop path distances are much
shorter. But to achieve the maximum distance in single-hop sporadic-E
propagation (about 1400-1500 miles in most cases), you still need a take-off
angle of as close to zero as you can get. Same for maximum-distance
single-hop F2 -- but the path distance there is much longer because the
refracting layer is so much higher.

As sporadic-E ionization intensity increases, the critical angle (at which
50 MHz refraction is possible) increases, allowing for much shorter
propagation paths. Under these conditions, a higher take-off angle becomes
advantageous. But the same is true for F2 propagation when trying to work a
short-skip F2 path (for example, about 1,500 miles)! The laws of physics are
the same in all cases.

>  Plus this is a 3 element beam he was asking about not a long boom
> so no matter what the take off angle will not be very low.

Actually, the ANGLE (not the strength) of the main lobe -- the one closest
to ground -- is a function mainly of the ambient terrain and the height of
the yagi above that terrain. Notwithstanding the anomalous effects of nearby
man-made objects (like other antennas), this is the same regardless of the
length of the antenna. Given identical ambient terrain, identical frequency,
and an identical placement of the antenna, the angle of the main (lowest)
lobe is going to be about the same.

What does change with boom length is the PROPORTION of the RF energy put int
o that main lobe compared with the higher-angle lobes. This is why longer
beams have more gain -- because they divert energy away from the
higher-angle lobes and concentrate them in the lowest-angle main lobe. But
this doesn't change the ANGLE of the main lobe.

They don't explain this in the antenna ads, but the forward gain of a yagi
can only be specified relative to a particular take-off angle. At any other
take-off angle, the gain of the antenna will be different. You have to think
three-dimensionally. This is why a 3-element yagi on 6M will often do better
than a 7-element yagi at the same height FOR THE SHORTER PATHS (say 400-600
miles on sporadic-E) -- because the smaller antenna puts more of its energy
into the higher-angle lobes, which the shorter propagation path needs. But
the 7-element antenna will do much better over the longer paths (1000-1500
miles on sporadic-E).

> Over the years I have had very good luck with my 5 element cushcraft
> only mounted 5 feet off the roof . . . and luckily my location allows me
> to have a low takeoff angle to work well with F2.

That same low take-off angle would allow you to work longer single-hop
sporadic-E paths as well. No difference except the actual distance worked.

Experimenting with 6M antennas is tremendously fun. Because of the shorter
wavelength, you can vary the distance (in wavelengths) above ground over a
wider range than is usually possible with HF antennas. Install HFTA (from
the ARRL Antenna Book 20th Edition and later), and see how well it reflects
reality. My experience: Pretty darn well!

73
Bill W5WVO